The outer cell membrane and the membranes surrounding inner cell organelles are bilipid layers. In a real cell the membrane phospholipid molecules create a spherical three dimensional lipid bilayer shell around the cell. However, they are often represented two-dimensionally as:

Each represents a phospholipid. The circle, or head, is the negatively charged phosphate group and the two tails are the two highly hydrophobic fatty acid chains of the phospholipid. Due to their thermodynamic properties they spontaneously form a double layer in an aqueous (watery) environment. In the human body all cells are surrounded by a thin layer of extracellular (means outside the cell) fluid which is mostly water. Basically what the molecules want to do in water is to point their heads to the water and their tails away from the water.

Model of a phospholipid molecule

There are two ways in which such molecules can be arranged. One is to form a micelle (or ball of molecules) with the heads forming the outer surface of the ball and the tails filling the space inside the ball.

 Diagrammatic representation of a micelle formed by phospholipid molecules

In the formation of a bilipid layer the tails of the phospholipids orient towards each other creating a hydrophobic environment within the membrane. This leaves the charged phosphate groups facing out into the hydrophilic environment.

The molecular forces which hold the molecules together in this way are known as Van der Waals bonds.

The membrane is approximately 5 nanometres thick.(a nanometre is a billionth of a metre thick)

The picture above is an electron micrograph of a cell membrane at approx. 240,000x magnification

This picture shows the cell membrane at a much lower magnification. The lighter area to the right represents the extracellular space.

This basic cell membrane structure has some very important features.

• it is fluid in nature allowing cell mobility. Note as mentioned above the physical properties of the bilipid layer mean that left to itself the shape of the cell will be spherical. However the fluid nature of the membrane means that the huge variety in cell shapes found in nature is possible.
• this bilipid layer is variably permeable, meaning that some molecules are allowed to pass freely (diffuse) through the membrane. The lipid bilayer is virtually impermeable to large molecules, relatively impermeable to molecules as small as charged ions such as Na⁺ and K⁺, and quite permeable to lipid soluble low molecular weight molecules such as ethanol (the organic chemical which gives alcoholic beverages their properties). The layer is extremely permeable to water molecules, but it is not well understood why. Molecules that can diffuse through the membrane do so at differing rates depending upon their ability to enter the hydrophobic interior of the membrane bilayer.
• any breaks or ruptures of the cell membrane are spontaneously repaired due to the molecular properties of the bilipid layer wanting to eliminate any free edges when in contact with water.
• studded throughout or associated with the bilipid layer there are various membrane proteins which perform various functions such as enzyme activity, cell attachment, communicating with other cells and transport various substances into and out of the cell.

The Fluid Mosaic Model

Lipid bilayers are fluid, and individual phospholipids diffuse rapidly throughout the two dimensional surface of the membrane. This is known as the fluid mosaic model of biological membranes (mosaic because it includes proteins, cholesterol, and other types of molecules besides phospholipids). The phospholipids can move to the opposite side of a bacterial cell membrane in a few minutes at room temperature. That is a distance several thousand times the size of the phospholipid. Membrane proteins diffuse throughout the membrane in the same fashion, though at a slower pace because of their relatively massive size compared to a phospholipid molecule. .

Notice that there are molecules of cholesterol embedded in the membrane. Cholesterol is a necessary component of biological membranes. Cholesterol breaks up the Van der Waals interactions and close packing of the phospholipid tails. This disruption makes the membrane more fluid. Therefore, one way for a cell to control the fluidity of its membrane is by regulating its level of cholesterol in the cell membrane.

Another way for the cell to control the fluidity of its membrane is to regulate the ratio of saturated to unsaturated hydrocarbon chains of the phospholipids. Saturated hydrocarbons are straight-chains ("saturated" with hydrogens), and unsaturated hydrocarbons have one or more double bonds (not "saturated" with hydrogens). A group of phospholipids with saturated hydrocarbon chains can pack close together and form numerous Van der Waals bonds that hold the phospholipids to each other. Phospholipids with unsaturated hydrocarbon side chains break up those Van der Waals bonds and the tight packing by preventing the phospholipids from getting close together.

The membrane proteins shown in the diagram above carry out most of the specialised functions of the cell membrane.

There are many types of membrane protein and these vary from cell type to cell type. It is important to remember that they can move around the cell membrane due to the fluid nature of the whole structure. They do not sit in one place but can move to where their function is required.

Membrane proteins can be

• Integral proteins : these are fully incorporated into the membrane and are in contact with both the inside and the outside of the cell.
• Surface membrane proteins : these are only associated with the outer of the bilipid layers and make contact with the extracellular space.
• Inner membrane proteins : these are only associated with the inner of the bilipid layers and make contact with the cytoplasm (cytosol) [one of these is shown to the right of the above diagram]
• Transmembrane channel proteins : these are similar to integral proteins but appear to possess a channel connecting the extracellular space to the cytoplasm.

The various functions of membrane proteins can be:

• to attach parts of the cytoskeleton to the cell membrane in order to provide shape.
• to attach cells to an extracellular matrix in grouping cells together to form tissues.
• to transport molecules into and out of cells by such methods as ion pumps, channel proteins and carrier proteins. [these mechanisms will be explored in detail later in the course and are of importance in understanding how some drugs and medicines act upon the body].
• to act as receptors for the various chemical messages which pass between cells such as nerve impulses and hormone activity.
• to take part in enzyme activity which can be important in the metabolism or as part of the body's defence mechanisms.

This page last updated Sat Aug 14 14:42:30 BST 2004

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